Voltage to Current Converter (V to I Converter)

Contents

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Key learnings:

Voltage to Current Converter Definition: A voltage to current converter is defined as an electronic circuit that converts an input voltage into a proportional output current.
Purpose of V to I Converters: These converters are essential in creating stable DC current signals for analog representation of physical quantities, ensuring less noise and constant current flow.
Ohm’s Law and Basic V to I Conversion: A resistor in a voltage source circuit can act as a simple V to I converter based on Ohm’s Law, though it may be imperfect.
Op-Amp Based Converters: Operational amplifiers like IC LM741 are used for more precise voltage to current conversion by maintaining the desired current through feedback mechanisms.
Types of V to I Converters: There are floating load converters (current series negative feedback amplifier) and ground load converters (Howland Current Converter), each with unique configurations and applications.

What is a Voltage to Current Converter (V to I Converter)?

A voltage to current converter (V to I converter) is defined as an electronic circuit that converts an input voltage into a proportional output current.

The motivation comes from instrumentation practice.

For instrumentation circuits, DC current is preferred for creating an analog representation of physical quantities like weight, pressure and motion.

DC current signals remain constant throughout the circuit from source to load, and current-sensing instruments generate less noise.

So sometimes a circuit must create current that corresponds to, or is proportional to, a definite voltage.

For this purpose Voltage to Current Converters (also known as V to I converters) are used. They simply change the carrier of electrical data from voltage to current.

Simple Voltage to Current Converter

When we discuss the connection between voltage and current, the natural starting point is Ohm’s law.

When a voltage is supplied as input to a circuit comprising a resistor, a proportional current flows through it.

The resistor therefore decides the current flow in a voltage source circuit; it performs as a simple voltage to current converter (that is, a V to I converter) for a linear circuit.

The circuit diagram below shows a resistor acting as a simple voltage to current converter. In this diagram, voltage and current are represented by bars and loops respectively.

In practice, however, the output current of this converter depends directly on the voltage drop across the connected load in addition to the input voltage. Since V_R becomes , this circuit is said to be an imperfect one, a bad or passive version.

Voltage to Current Converter Using Op-Amp

An op-amp is implemented to convert the voltage signal into a corresponding current signal. The op-amp used for this purpose is IC LM741.

This op-amp holds the precise amount of current by applying whatever voltage is needed to sustain that current throughout the circuit. Two types exist, explained in detail below.

Floating Load Voltage to Current Converter

As the name indicates, the load resistor floats in this converter circuit: R_L has no connection to ground.

The input voltage V_IN is applied to the non-inverting input terminal. The inverting input terminal is driven by the feedback voltage developed across the R_L resistor.

This feedback voltage is determined by the load current and sits in series with the input difference voltage (VD). The circuit is therefore also known as a current-series negative feedback amplifier.

For the input loop, the voltage equation is

Since A is very large,
So,

Since the input to the Op-amp,

From the above equation it is clear that the load current depends on the input voltage and the input resistance.

That is, the load current,, follows from the input voltage alone. The load current is controlled by the resistor R, whose inverse (1/R) serves as the proportionality constant.

This converter circuit is therefore also known as a Trans-Conductance Amplifier. Its other name is Voltage Controlled Current Source.

The load itself may be resistive, capacitive or non-linear; its type plays no role in the above equation.

When the connected load is a capacitor, it charges or discharges at a steady rate. For this reason, the converter circuit is used for producing sawtooth and triangular waveforms.

Ground Load Voltage to Current Converter

This V to I converter is also known as the Howland Current Converter. Here, one end of the load always remains grounded.

For circuit analysis we first determine the voltage V_IN; the relationship between the input voltage and the load current then follows.

To obtain it, we apply Kirchhoff’s current law at node V₁:

For a non-inverting amplifier, the gain is
Here, the resistor, .
So, . Hence the output voltage will be

We can conclude from the above equation that the current I_L relates to the voltage V_IN and the resistor R.